Hi Big Fish... I am still in the learning process. So I am not guru, actually took tips from the fellow forumer here too.
Back to topic BigFish. If you opted out of OCS, they will give you a RAW concrete flooring. It's uneven and got spikes & bumps here and there. You cannot perform flooring of any form.
Option 1:
Go to the Sembawang HBD branch, which is just opposite the MRT station, below is the NTUC mart, so you probably can't miss that building. Signed up the form, and they will contact you when they will come and smooth out and flatten your floor. The person will tell you to wait for 3~4 weeks. But I got the lastest news is that they will do it this Saturday. So it's only 2 1/2 weeks, but HDB as HDB, always cover arse even it is not necessary.
Option 2: Get contractors to do it. Contractors can do it immediately but at a cost of $1000.00. Basically from my contractor, he says not much difference, unless you are desperate to move in ASAP, you scarifice money for time.
But if you calculate your reno timing well... you contractors can start kitchen, toilet, aircon piping and etc. Getting HDB to do your smoothing and flattening doesn't affects much.
When you get your keys, your contractor and you will walk the ground and prepare the things you want and identify the things you need. Drawings and negotiation will take some time. This will happen even for ID. Dun forget, usually aircon piping will run in the ceiling and get conceal by L box if you have any and usually, it is better to run the pipes first before the flooring.